The present disclosure relates generally to systems and methods for monitoring nerve activity and, in particular, to systems and methods for non-invasive monitoring of nerve activity with cutaneous and subcutaneous electrodes.
Many diagnostic and treatment methods in the fields of medicine and biology rely on measurements of nerve activity in patients and test subjects. Nerve activity in humans and other animals generates electrical signals that are detectable by electronic equipment such as oscilloscopes and other electrical signal processing devices. In order to detect the nerve activity, one or more electrical conductors, or electrodes, are placed in proximity to the nerves being measured. The electrodes may receive the electrical signals for further medical analysis. Various medical treatment methods also use electrodes to deliver electrical signals to the nerves in order to induce a response in the patient.
Cardiac care is one particular area of medical treatment that heavily utilizes measurement of nerve activity. Activity in the autonomic nervous system controls the variability of heart rate and blood pressure. The sympathetic and parasympathetic branches of the autonomic nervous system modulate cardiac activity. Elevated levels of sympathetic nerve activity (SNA) are known to be correlated with heart failure, coronary artery disease, and may be associated with the initiation of hypertension. SNA is also thought to be important as a predictor of heart rhythm disorders, including sudden cardiac death. Therefore, a diagnostic index of “autonomic tone” produced in accordance with measurement of SNA may have considerable clinical value. As known in the art, clinical utilization of autonomic nerve activity is mostly derived from biochemical perturbations like the use of beta-blockers in high blood pressure management. While elevated levels of SNA are known to be correlated with these medical conditions, more precise analysis of the particular electrical signals produced by sympathetic nerves is needed before sympathetic nerve measurement can become a useful diagnostic or prognostic tool. Deficiencies in current technology result in either poor autonomic signal quality or present some difficulty in integrating implantable electronic enhancements (like telemetry, on-chip amplification, storage memory, and motion sensors).
One challenge to measuring nerve activity is that the magnitude of electrical signals in the sympathetic nerves is relatively low, while various other electrical signals present in a patient provide noise that may interfere with isolation and detection of the sympathetic nerve activity. For example, in the human body and the bodies of many animals the electrical activity in the cardiac muscle generates electrical signals with much greater amplitudes than the amplitudes of electrical signals in the nerves. Other muscles in the body can also generate large electrical signals, but the cardiac muscle contractions in a heartbeat occur continuously during any nerve monitoring procedure, and the electrical signals from the cardiac muscle contractions present difficulties in monitoring the lower amplitude signals in the nerve fibers.
In the existing art, a doctor or healthcare professional performs a microneurography procedure on a patient to monitor nerve activity. In microneurography, one or more metal or glass electrodes are inserted into the body of a patient in close proximity to a nerve fiber bundle. The electrodes are formed as thin needles, and the doctor places the needle tip of each electrode in close proximity to the nerve bundle for precise monitoring of the electrical activity in the nerve bundle. The placement of the electrode in close proximity to the nerve enables the electrodes to detect electrical signals that are generated due to nerve activity and to distinguish the nerve activity from the larger electrical signals in the body due to, for example, the cardiac muscle activity. The electrodes receive electrical signals corresponding to the nerve activity in the nerve fiber bundle, and the electrical signals propagate from the electrodes through electrical leads for display and processing using electronic monitoring equipment. Microneurography is an invasive procedure because the electrodes are inserted into the body of the patient. In some scenarios, a doctor punctures the skin of the patient with the needle electrodes to monitor some nerve fibers that are near the surface of the body. In situations where the nerves to be monitored are located deeper within the body, the doctor must perform surgery to implant the electrodes.
While microneurography is effective at monitoring some types of nerve activity, the procedure includes several drawbacks. Because microneurography is an invasive procedure, the patient is typically immobilized to prevent damage to the electrodes, injury to the patient, and to maintain the position of the electrodes in close proximity to the nerve fiber during the monitoring process. During microneurography, a doctor or medical professional inserts the electrodes and removes the electrodes after a relatively short monitoring period, which precludes long-term monitoring of nerve activity and requires that the patient be present in a hospital or other medical facility for nerve monitoring. Additionally, microneurography is not suitable for monitoring many nerves in human patients because the nerves are located in inaccessible regions of the body. For example, microneurography is not approved for use in humans for monitoring nerve fibers, such as the stellate ganglia, which are proximate to the heart, although microneurography is used in studies of cardiac nerve activity in animal test subjects.
Medical professionals, scientists, and patients have a need to monitor nerve activity in a less invasive manner than prior art techniques, such as microneurography, and to perform nerve monitoring in situations where microneurography is impractical. Consequently, improvements to systems and procedures for monitoring nerve activity in human and animal patients would be beneficial.